Carbon steel bars having good machinability may be produced by the addition of certain elements, commonly phosphorous, lead, sulfur, or certain rare earth elements, either singly or in combination. The present invention relates to the addition of certain of these elements in the powdered form of a naturally occurring compound.
In the known method of adding lead to steel, a stream of high-purity, closely sized (-20, +40 mesh) lead shot is introduced by means of an air-blast gun into the stream of molten steel as it is being teemed from ladle to mold. The pressure of the gun is carefully controlled so that the rate of addition is proportional to the teeming rate. Extreme care is required to minimize lead segregation. Because lead has only limited solubility in molten steel, because of the greater density of lead, and because of the lower solidus temperature of lead, significant lead segregation frequently occurs--especially at the bottom of the ingot molds in conventional casting.
In conventional casting, a portion of the bottom of the ingot is cut off and discarded; a test portion of the remaining ingot bottom is then removed and given a sweat test for exudation of lead as determined by visual inspection. Further sections may be removed until the sweat test indicates acceptable segregation levels. The lead is generally conceded to be present in elemental form as small inclusions, though previously it was believed to exist as a submicroscopic dispersion. The lead inclusions usually accompany manganese-sulfide inclusions in manganese deoxidized, resulfurized and leaded steels.
It is believed that the lead inclusions improve free-machinability due to softness and an internal lubricity effect. The optimum shape for the lead and/or manganese sulfide inclusions is believed to be round, or stated differently, as having a minimal aspect ratio. Free machinability is believed associated with this minimum aspect ratio and with well dispersed inclusions.
Silicon and aluminum are to be minimized or avoided, as manganese silicate inclusions impair machinability because they are abrasive, have an undesirable stringer morphology, and surround the manganese sulfide inclusions, thus reducing their effectiveness. Abrasive inclusions such as alumina, produced by aluminum deoxidation practices, produce rapid wear of cutting tools and are thus also to be avoided.